Reflection type terahertz spectrometer and spectrometric method

ABSTRACT

A reflection-type terahertz spectrometer includes an input optical path through which terahertz waves are propagated, an irradiating mechanism that irradiates a sample with terahertz waves propagated through the input optical path, an output optical path through which terahertz waves exiting from the irradiating mechanism are propagated, and a detector that receives and detects the terahertz waves propagated through the output optical path. The irradiating mechanism has at least one planar interface and a refractive index greater than that of a peripheral region contacting the planar interface and is disposed between the input optical path and the output optical path such that the terahertz waves propagated through the input optical path to be incident on the planar interface undergo total internal reflection at the planar interface, and the sample is disposed in the peripheral region contacting the planar interface of the irradiating mechanism. When the terahertz waves undergo the total internal reflection at the planar interface, the sample is irradiated with evanescent waves scattering from the planar interface to the peripheral region contacting the planar interface, so as to measure a spectrum.

TECHNICAL FIELD

The present invention relates to an instrument and a method whichmeasures a spectrum of a sample under analysis in a terahertz frequencyrange.

BACKGROUND ART

Terahertz waves are electromagnetic waves with frequencies between 0.1and 10 THz (wavelengths between 30 μm and 3000 μm). This wavelengthrange approximately corresponds to the infrared to far infrared region.Terahertz spectrometers using these terahertz waves have been developed.

The terahertz spectrometers are divided into a transmission type whichirradiates terahertz waves on a sample and detects transmitted light,and a reflection type which irradiates terahertz waves on a sample anddetects reflected light. The transmission-type spectrometers needsamples to be in the form of a thin film of about 1 μm, since terahertzwaves, which fall in the infrared to far infrared region, are stronglyabsorbed by most substances. Therefore, much attention has been paid tothe reflection-type spectrometers which do not limit the thickness ofsamples.

As shown in FIG. 7, a conventional reflection-type spectrometergenerates terahertz waves by pumping InAs 30 by an ultrashort pulselaser, irradiates the terahertz waves on a sample 35 by way of off-axisparabolic mirrors 31, 34, and makes reflected light incident on aphotoconductive dipole antenna 37 by off-axis parabolic mirrors 34, 36for photoelectric detection (See Kiyomi SAKAI et al, “Terahertz TimeDomain Spectroscopy and Imaging”, The Review of Laser Engineering, Vol.30, No. 7, July 2002, pp. 376-384). In this conventional reflection-typespectrometer, first light reflected by the sample 35 isphotoelectrically detected and next, light reflected by a metallicmirror which is placed in the same position as the sample 35 is alsophotoelectrically detected for reference. Then complex amplitudes in thefrequency domain obtained by computing the Fourier transform of therespective detected photoelectric waveforms are compared with each otherso as to derive reflectivity and phase shift. The most important problemof this spectrometer is that an error occurs in phase shift unlessmeasurement is conducted with the metallic mirror and the sample placedin exactly the same position. Besides, its samples are limited tosolids. And liquid, amorphous living organisms or the like cannot bemeasured.

As mentioned above, the conventional reflection-type terahertzspectrometer requires the sample and the metallic mirror to be placed inthe same position and has made large measurement errors. To decrease themeasurement errors, these two objects need to be placed in the sameposition with high accuracy. This placement takes a lot of time and thisspectrometer is poor in practicality. In addition, since its samples arelimited to solids, this spectrometer is poor in general versatility.

The present invention has been made in view of the above problems of theconventional reflection-type terahertz spectrometer. It is an object ofthe present invention to provide a reflection-type terahertzspectrometer and spectrometric method in which a metallic mirror doesnot have to be placed in the same position as a sample and samples arenot limited to solids.

DISCLOSURE OF THE INVENTION

The reflection-type terahertz spectrometer of the present inventioncomprises an input optical path through which terahertz waves arepropagated, an irradiating means which irradiates a sample with theterahertz waves propagated through the input optical path, an outputoptical path through which terahertz waves having exiting from theirradiating means are propagated, and a detecting means which receivesand detects the terahertz waves propagated through the output opticalpath, and is characterized in that the irradiating means has at leastone planar interface and a refractive index greater than that of aperipheral region contacting the planar interface and is disposedbetween the input optical path and the output optical path such that theterahertz waves propagated through the input optical path to be incidenton the planar interface undergo total internal reflection at the planarinterface, and the sample is disposed in the peripheral regioncontacting the planar interface of the irradiating means, and when theterahertz waves undergo the total internal reflection at the planarinterface, the sample is irradiated with evanescent waves scatteringfrom the planar interface to the peripheral region contacting the planarinterface, so as to measure a spectrum.

The evanescent waves scattering from the planar interface of theirradiating means play interaction with the sample and the terahertzwaves including information of the evanescent waves have exited from theirradiating means to the output optical path. In the absence of a samplein the neighborhood of the planar interface, terahertz waves includinginformation of evanescent waves which do not play interaction with thesample have exited, so these terahertz waves in the absence of a samplecan be used for reference. There is no need to conduct referencemeasurement by placing a metallic mirror. In addition, owing to the useof the interaction between the evanescent waves scattering from theplanar interface of the irradiating means and the sample, samples arenot limited to solids.

This terahertz spectrometer can further comprise a polarization controlmeans which controls polarization of the terahertz waves in the midst ofthe input optical path, so as to control polarization of the evanescentwaves.

By employing this means, the interaction between longitudinal waves ortransverse waves and the sample can be selectively played and it becomespossible to observe absorption spectrum by plasma, longitudinal phononsin a semiconductor, etc. First, when the terahertz waves are p-polarizedby the polarization control means and are made incident on theirradiating means, the evanescent waves scattering from the planarinterface are subjected to longitudinal wave modulation, thereby havingboth a longitudinal wave component and a transverse wave component.Second, when the terahertz waves are s-polarized by the polarizationcontrol means and are made incident on the irradiating means, evanescentwaves having a transverse wave component alone are obtained. Thelongitudinal wave component can be extracted by differentiating thesetwo waves.

The irradiating means can be formed of one of silicon, germanium,diamond, III-V semiconductors including GaAs, II-VI semiconductorsincluding ZnSe, silica glass, fluororesin, polyethylene, andpolycarbonate-containing organic materials.

By thus forming the irradiating means, terahertz wave absorption lossinside the irradiating means can be decreased.

The reflection-type terahertz spectrometer can further comprise ahousing with an opening for accommodating the input optical path and theoutput optical path, and the irradiating means may be disposed so as toclose the opening with the planar interface of the irradiating means.

Due to their long wavelengths, terahertz waves are strongly absorbed byatmospheric H₂O, etc., which becomes noise in terahertz spectrometry.However, the inner space of the housing can be purged with nitrogen,etc. or vacuumed, and as a result absorption by H₂O, etc. can beprevented.

A thin film which has a refractive index smaller than that of theirradiating means and does not absorb terahertz waves can be formed onthe planar interface of the irradiating means.

Owing to this arrangement, terahertz waves can be totally internallyreflected at the planar interface of the irradiating means which is incontact with the thin film, and at the same time evanescent waves canscatter from the planar interface to the thin film. Therefore, as longas the thin film has a sufficiently small thickness, terahertzspectrometry can be performed even when a sample having a refractiveindex larger than that of the irradiating means is disposed on this thinfilm.

A reflection-type terahertz spectrometric method of the presentinvention for dissolving the above mentioned problems, which measures aspectrum of a sample in a terahertz wavelength region by irradiatingterahertz waves on the sample and detecting reflected waves from thesample by a detecting means, is characterized by placing, in the midstof an optical path between the generating means and the detecting means,an optical medium having a refractive index larger than that of theoptical path such that terahertz waves incident on the optical mediumundergo total internal reflection at an interface of the optical mediumso as to generate evanescent waves from the interface, and by placingthe sample in the neighborhood of the interface of the optical medium soas to irradiate the evanescent waves on the sample and measure aspectrum.

In the above method, polarization of the evanescent waves can becontrolled by polarizing the terahertz waves from the generating meansto be incident on the optical medium by a polarization control means.

Moreover, when polarization of the evanescent waves is controlled asabove, the reflection-type terahertz spectrometric method of the presentinvention can measure a spectrum by p-polarizing the terahertz wavesfrom the generating means to be incident on the optical medium by thepolarization control means, next measure a spectrum by s-polarizing theterahertz waves from the generating means to be incident on the opticalmedium by the polarization control means, and differentiate these twospectra.

The optical medium used in the terahertz spectrometric method of thepresent invention can be formed of one of silicon, germanium, diamond,III-V semiconductors including GaAs, II-VI semiconductors includingZnSe, silica glass, fluororesin, polyethylene, polycarbonate-containingorganic materials.

Moreover, in the method of the present invention, the optical path andthe sample can be spatially isolated from each other at the interface ofthe optical medium.

Furthermore, in the method of the present invention, the optical mediumcan be provided, at the interface, with a thin film which has arefractive index smaller than that of the optical medium and does notabsorb the terahertz waves such that the terahertz waves undergo totalinternal reflection at the interface on which the thin film is formed.

When the reflection-type terahertz spectrometer of the present inventionis used in carrying out the aforementioned reflection-type terahertzspectrometric method of the present invention, the irradiating means canbe used as the optical medium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the structure of a reflection-typeterahertz spectrometer according to a first preferred embodiment.

FIG. 2 is a partial view of FIG. 1, showing that total reflection occursat a planar interface in the reflection-type terahertz spectrometer ofthe first preferred embodiment.

FIG. 3 shows the spectra of radiation (in the presence and absence of asample) measured by the reflection-type terahertz spectrometer of thefirst preferred embodiment.

FIG. 4 shows a spectrum obtained by dividing the spectrum of theradiation in the presence of the sample with the spectrum of theradiation in the absence of the sample.

FIG. 5 is a schematic diagram showing the structure of a reflection-typeterahertz spectrometer according to a second preferred embodiment.

FIG. 6 is a schematic diagram showing the structure of a reflection-typeterahertz spectrometer according to a third preferred embodiment.

FIG. 7 is a schematic diagram showing the structure of a conventionalreflection-type terahertz spectrometer.

BEST MODE FOR CARRYING OUT THE INVENTION

Terahertz waves can be generated by irradiating a photoconductivesemiconductor switch (i.e., a low-temperature-grown GaAs (LT-GaAs)having a metallic antenna thereon), a surface of a bulk semiconductorsuch as InAs, a semiconductor quantum well, a nonlinear optical crystal,a high-temperature superconductor, etc. with ultrashort light pulseshaving a duration of not more than 100 fs from a mode-locked titaniumsapphire laser or the like and providing pumping. Generally, ultrashortlight pulses are focally irradiated in order to increase the efficiencyand intensity of terahertz waves generating. The photoconductive switchhas a high efficiency of generating terahertz waves, but is destructedwhen irradiated with a strong laser for an increase in the efficiency ofgenerating terahertz waves, and degrades with the passage of time. InAsshows the highest efficiency of generating terahertz waves among bulksemiconductors, and exhibits an increase in the efficiency and intensityof terahertz waves generating especially upon the application of astrong magnetic field. It is to be noted that terahertz waves can alsobe generated by irradiating a LT-GaAs photoconductive switch with two cwsingle-mode semiconductor lasers or cw multi-mode semiconductor lasershaving similar emission wavelengths. This has a merit of not requiringan expensive femtosecond laser.

The input optical path through which terahertz waves generated by theterahertz wave generating means are propagated is a terahertz wavepassage from the terahertz wave generating means to the subsequentirradiating means, and can be open space or a space closed by a bodytube, etc. A closed space is preferable because absorption by H₂O, etc.can be prevented, for instance, by purging the inner space withnitrogen. In measuring a small sample, a condenser optical system ispreferably provided in the midst of the input optical path. This enablesmeasurement of small samples. The reason why the condenser opticalsystem is required is as follows: Terahertz waves are generated, asmentioned above, by focally irradiating InAs, etc. with ultrashortoptical pulses, so the terahertz waves generated from the terahertz wavegenerating means are generally spherical waves. Therefore, it ispreferable to collimate these terahertz waves once by an off-axisparabolic mirror or the like and focus the collimated terahertz waves byanother off-axis parabolic mirror or the like so as to make theterahertz waves incident on the irradiating means. Then the intensity ofthe terahertz waves can be increased and spectrum detection sensitivitycan be enhanced. Moreover, when a polarization control means whichcontrols polarization of the terahertz waves is additionally provided inthe midst of the input optical path, the polarization control means canbe a sheet polarizer, which is formed by drawing fluororesin,polyethylene, polycarbonate-containing organic materials, etc. in onedirection, a wire grid polarizer, and so on.

The irradiating means has only to have at least one planar interface anda refractive index larger than that of a peripheral region contactingthe planar interface. The irradiating means can be in the shape of ahalf-cut cylinder, a hemisphere, a triangular prism, etc. The materialof the irradiating means is preferably silicon, germanium, diamond,III-V semiconductors including GaAs and II-VI semiconductors includingZnSe, silica glass such as fused silica and crystal quarts, fluororesin,polyethylene, or polycarbonate-containing organic materials. This isbecause terahertz wave absorption loss inside the irradiating means canbe small. The III-V and II-VI semiconductors have a small terahertz waveabsorption loss inside themselves but have a strong reflection at theinterface because of their large refractive indices, so the III-V andII-VI semiconductors may have an antireflection film, etc. on the planeof incidence and the plane of exit, if necessary. The use of silicaglass, polyethylene, etc. enables a change in soft materials undervisible light irradiation and pump-probe spectroscopy, which is a typeof time-resolved spectroscopy, owing to their transparency even to thevisible light (i.e., small absorption loss). As the fluororesin, it isparticularly preferable to employ polytetrafluoroethylene. The use ofpolytetrafluoroethylene enables spectra of samples including acid oralkali to be measured, since polytetrafluoroethylene is especiallyresistant to acid or alkali. Besides, if the planar interface of theirradiating means is put upward, a spectra can be measured simply byplacing a sample on the planar interface. Therefore, powdery chemicalsand organic functional materials such as DNA in liquid can be used assamples. Moreover, bulk superconductors, which cannot be measured byconventional reflection-type spectrometers because of their largerefractive indices, can be used as samples. If polarization of theevanescent waves is controlled by controlling polarization of theterahertz waves incident on the irradiating means, even influence ofJosephson plasma (longitudinal waves), which is associated withsuperconductivity phenomena of high-temperature superconductors, isdetectable and high-temperature superconductors can also be used assamples.

The irradiating means can be formed of the abovementioned materials anda thin film having a refractive index smaller than that of theirradiating means can be formed on its planar interface. When theirradiating means is formed of silica, the thin film can be formed ofpolyethylene, for instance. This thin film needs to have such a smallthickness that evanescent waves can reach a sample placed on the thinfilm. This can be achieved by the film thickness of about 1 μm.

A sample under analysis is disposed in a peripheral region contactingthe planar interface of the irradiating means. The peripheral region isa region where evanescent waves from the planar interface scatter and isof the order of terahertz wavelength, namely, in the range of 3 μm to300 μm from the planar interface.

The output optical path through which terahertz waves having exitingfrom the irradiating means are propagated is a terahertz wave passagefrom the irradiating means to the subsequent detecting means, and can beopen space or a space closed by a body tube, etc. A closed space ispreferable because absorption by H₂O, etc. can be prevented, forinstance, by purging the inner space with nitrogen. A condenser opticalsystem is desirably provided in the midst of the output optical path. Itis preferable to collimate the terahertz waves which have undergonetotal internal reflection at the planar interface of the irradiatingmeans once by an off-axis parabolic mirror or the like and focus thecollimated terahertz waves by another off-axis parabolic mirror or thelike so as to make the terahertz waves incident on the detecting means.Then the intensity of the terahertz waves can be increased and spectrumdetectivity can be enhanced.

The detecting means can be a bolometer and a device using electro-optic(EO) effects such as a photoconductive antenna and ZnTe. The bolometercan convert terahertz waves into electric signals single-handedly but ispoor in responsibility. The device using electro-optic effect serves toirradiate EO crystal with terahertz waves to be detected and detect avariation in refractive index induced by the electric field of theterahertz waves by means of a variation in the polarization of the probelight traveling through the EO crystal. This method using the EO effectshas a merit of being capable of measuring time waveform, namely, phaseinformation, which cannot be measured by the bolometer. The variation inthe polarization of the probe light can be detected by a polarizationcontrol element and a photoelectric detection element. For example,detection can be carried out by converting the probe light transmittedthrough the EO crystal into linearly polarized light by a ¼ wave plate,splitting that linearly polarized light by a polarization beam splitter,detecting the split lights by two photodiodes, etc., and inputting twoelectric signals into a balance detector.

The housing with an opening for accommodating the input optical path andthe output optical path has only to be sealed when the opening isclosed. Its material is not limited, but, if the inner space needs to bevacuumed, stainless steel is desirable. This is because no gasgeneration occurs. When a laser light source irradiating the terahertzwave generating means is disposed outside the housing, the housing hasto be provided with a laser introducing window for irradiating thegenerating means with a laser from the laser light source.

First Preferred Embodiment

FIG. 1 is a schematic view showing the structure of a reflection-typeterahertz spectrometer according to a preferred embodiment of thepresent invention.

The spectrometer of this preferred embodiment comprises an input opticalpath 1 through which terahertz waves from a terahertz wave generatingmeans 13 are propagated, an irradiating means 2 which irradiates asample 5 with the terahertz waves propagated through the input opticalpath 1, an output optical path through which terahertz waves havingexiting from the irradiating means 2 are propagated, and a detectingmeans 4 which receives and detects the terahertz waves propagatedthrough the output optical path 3.

The terahertz wave generating means 13 is a bulk semiconductor InAs inwhich ultrashort light pulses with a wavelength of 800 nm, a pulse widthof 100 fs, a repetition of 80 MHz from a femtosecond laser light source10 are split in half by a polarization beam splitter 11, bent by bendmirrors 8, 8′ and then focally irradiated by a lens 12. As shown in FIG.1, the InAs 13 is applied with a magnetic field of 1 tesla by a magnet(not shown) in a direction perpendicular to the plane of incidence (theplane of the page). Though the terahertz wave generating means 13 is apart of the spectrometer of this preferred embodiment, the terahertzwave generating means 13 can be provided independently from thespectrometer.

On the input optical path 1, there are inserted an off-axis parabolicmirror 14 for collimation and an off-axis parabolic mirror 15 forfocused radiation. The off-axis parabolic mirrors 14, 15 have a focallength of 200 mm.

The irradiating means 2 is an isosceles prism with a vertex angle of130° formed of polytetrafluoroethylene (refractive index n=1.45). Theside opposite to the vertex becomes a planar interface 21 and the prism2 is disposed in the air (refractive index n₀=1.0) between the inputoptical path 1 and the output optical path 3 such that the terahertzwaves are incident from one side 22 of the isosceles sides and exit fromthe other side 22′. As shown in FIG. 2, the terahertz waves incident onthe side 22 in a parallel direction to the planar interface 21 have anincident angle of 65° to the side 22. The refraction angle α is:α=sin⁻¹[(1/1.45)sin 65°]=38.7°.The incident angle β to the planar interface 21 is:β=63.7°.The critical angle θc, at which total reflection occurs, is:θc=sin⁻¹(1/1.45)=43.6°Since the incident angle β is greater than the critical angle θc, totalreflection occurs. The terahertz waves totally reflected travel insymmetry with the incident waves from the total reflection point andexit from the side 22′, since the irradiating means 2 is an isoscelesprism.

On the output optical path 3, there are inserted an off-axis parabolicmirror 16 for collimation and an off-axis parabolic mirror 17 forfocused radiation. The off-axis parabolic mirrors 16, 17 have a focallength of 200 mm. The paraboloidal mirror 17 has a hole through which ahalf of the ultrashort light pulses split by the polarization beamsplitter 11 pass.

The detecting means 4 comprises EO crystal ZnTe 41, a ¼ wave plate 42, apolarization beam splitter 43 and photodiodes 44, 44′. The terahertzwaves propagating through the output optical path 3 is focallyirradiated on the ZnTe 41 by way of the off-axis parabolic mirror 17. Inthe absence of terahertz waves, the ZnTe 41 does not have a variation inrefractive index, so linearly polarized ultrashort light pulses incidentthrough the hole of the off-axis parabolic mirror 17 become circularlypolarized light after passing through the ¼ wave plate 42. Hence, thep-polarized light component and the s-polarized light component areidentical with each other, so electric signals output from thephotodiodes 44, 44′ after split by the polarization beam splitter 43have the same value, and the balance (a difference between the twoelectric signals) measured by a balance detector 20 becomes zero. In thepresence of terahertz waves, the refractive index of the ZnTe 41 isvaried by the terahertz wave electric field, so polarization of thelinearly polarized ultrashort light pulses incident through the hole ofthe off-axis parabolic mirror 17 is rotated and they becomeelliptically-polarized light after passing through the ¼ wave plate 42.Hence, there is a difference between the p-polarized light component andthe s-polarized light component. Electric signals output from thephotodiodes 44, 44′ after split by the polarization beam splitter 43have different values and a balance is detected. This balance isproportional to the strength of the electric field of the terahertzwaves incident on the ZnTe. In order to match, on the ZnTe 41, thetiming of the terahertz waves irradiated focally by the off-axisparabolic mirror 17 and that of the ultrashort light pulses incidentthrough the hole of the off-axis parabolic mirror 17, the ultrashortlight pulses separated by the polarization beam splitter 11 aresubjected to delay control by a delay line which comprises a cat's-eyemirror 18 and bend mirrors 19, 19′.

Measurement was conducted by using dielectric strontium titanate (STO)as a sample 5 and pressing the STO 5 against the planar interface 21 ofthe prism 2, and the results are shown in FIGS. 3 and 4. FIG. 3 showsthe Fourier transform of the time variation of the terahertz waveelectric field detected by the balance detector 20 in the presence andabsence (for reference) of the STO 5 at the planar interface 21. Thespectra of the two are identical in a high frequency region but largelydifferent from each other in a low frequency region. It is to be notedthat most of the absorption lines are due to absorption by atmosphericH₂O. FIG. 4 shows the result of dividing the spectrum of the STO by thereference spectrum. In FIG. 4, the absorption lines due to H₂O disappearand absorption is apparent around 1.1 THz as indicated by the arrow.

Second Preferred Embodiment

FIG. 5 is a schematic diagram showing the structure of thereflection-type terahertz spectrometer according to a second embodimentof the present invention. Although the spectrometer of the firstpreferred embodiment was affected by absorption by atmospheric H₂O, thespectrometer of the second preferred embodiment is constructed so as notto be affected by absorption by H₂O. Hence, the spectrometer of thesecond preferred embodiment comprises a housing 6 which isolates theinput optical path 1, the irradiating means 2 and the output opticalpath 3 of the first preferred embodiment (shown in FIG. 1) from theoutside. In FIG. 5, the same constitutional elements as those of thefirst preferred embodiment are denoted by the same reference numerals asthose of FIG. 1 and repetition of description is omitted.

The housing 6 comprises an opening 61, windows 62, 62′, a gas inletvalve 63 and a gas exhaust valve 64, and the opening 61 is sealed withthe planar interface 21 of the irradiating means 2. The inner space ofthe housing 6 was purged with nitrogen by connecting a nitrogen gascylinder to the side of the inlet valve 63 and opening the exhaust valve64, and measurement was conducted by using the same STO as used in thefirst preferred embodiment as a sample 5. As a result, no absorption byH₂O was observed and the signal-to-noise ratio was greatly improved.

Third Preferred Embodiment

FIG. 6 is a schematic diagram showing the structure of a reflection-typeterahertz spectrometer according to a third preferred embodiment of thepresent invention. The spectrometer of this preferred embodiment isconstructed so as to be capable of observing an absorption spectrum byplasma, longitudinal phonons in a semiconductor, etc. Hence, thespectrometer of this preferred embodiment comprises a polarizer 7, whichis a polarization control means, on the input optical path 1 of thefirst preferred embodiment (shown in FIG. 1). In FIG. 6, the sameconstitutional elements as those of the first preferred embodiment aredenoted by the same reference numerals as those of FIG. 1 and repetitionof description is omitted.

A compound semiconductor GaAs was used as a sample 5. First theterahertz waves which were p-polarized by controlling the polarizer 7were made incident on the side 22 of the prism 2 and a spectrum wasmeasured. Next, the terahertz waves which were s-polarized by turningthe polarizer 7 at an angle of 90° were made incident on the side 22 anda spectrum was measured. Then a spectrum of a difference between thespectrum of the p-polarized light and the spectrum of the s-polarizedlight was calculated, and as a result, absorption was confirmed around 7THz. This corresponds with ν=7.2 THz, which was calculated from theseparately calculated phonon's energy E=h ν=0.03 eV, where h is Planck'sconstant, ν is phonon frequency.

INDUSTRIAL APPLICABILITY

As mentioned above, the reflection-type terahertz spectrometer andspectrometric method of the present invention is useful in measuring aspectrum in the frequency range from 0.1 to 10 THz (in the wavelengthregion from 30 μm to 3000 μm) by means of reflection, and suitable formeasuring absorption spectra of semiconductors and superconductors, etc.

1. A reflection-type terahertz spectrometer, comprising: an inputoptical path through which terahertz waves are propagated, anirradiating means which irradiates a sample with said terahertz wavespropagated through said input optical path, an output optical paththrough which terahertz waves having been exiting from said irradiatingmeans are propagated, a detecting means which receives and detects saidterahertz waves propagated through said output optical path, and apolarization control means which controls polarization of said terahertzwaves at said input optical path, wherein said irradiating means has atleast one planar interface and a refractive index greater than that of aperipheral region contacting said planar interface and is disposedbetween said input optical path and said output optical path such thatsaid terahertz waves propagated through said input optical path to beincident on said planar interface undergo total internal reflection atsaid planar interface, and said sample is disposed in said peripheralregion contacting said planar interface of said irradiating means, andwhen said terahertz waves undergo said total internal reflection at saidplanar interface, said sample is irradiated with evanescent wavesscattering from said planar interface to said peripheral regioncontacting said planar interface, so as to measure a spectrum.
 2. Thereflection-type terahertz spectrometer according to claim 1, whereinsaid irradiating means is formed of one of silicon, germanium, diamond,III-V semiconductors including GaAs and II-VI semiconductors includingZnSe, silica glass, fluororesin, polyethylene, orpolycarbonate-containing organic materials.
 3. The reflection-typeterahertz spectrometer according to claim 1, further comprising ahousing with an opening for accommodating said input optical path andsaid output optical path, said irradiating means being disposed so as toclose said opening with said planar interface.
 4. The reflection-typeterahertz spectrometer according to claim 1, wherein a thin film whichhas a refractive index smaller than that of said irradiating means anddoes not absorb said terahertz waves is formed on said planar interfaceof said irradiating means.
 5. A reflection-type terahertz spectrometricmethod, comprising: measuring a spectrum of a sample in a terahertzwavelength region by irradiating terahertz waves from a terahertz wavegenerating means on said sample and detecting reflected waves from saidsample by a detecting means, placing, within an optical path betweensaid generating means and said detecting means, an optical medium havinga refractive index larger than that of said optical path such thatterahertz waves incident on said optical medium undergo total internalreflection at an interface of said optical medium so as to generateevanescent waves from said interface, controlling, using a polarizationcontrol means, polarization of said evanescent waves by polarizing saidterahertz waves from said generating means to be incident on saidoptical medium, placing said sample in a vicinity of said interface ofsaid optical medium so as to irradiate said evanescent waves on saidsample and measure a spectrum.
 6. The reflection-type terahertzspectrometric method according to claim 5, wherein the measuringmeasures a spectrum by p-polarizing said terahertz waves from said wavegenerating means to be incident on said optical medium by saidpolarization control means, and next measures a spectrum by s-polarizingsaid terahertz waves from said wave generating means to be incident onsaid optical medium by said polarization control means anddifferentiating these two spectra.
 7. The reflection-type terahertzspectrometric method according to claim 5, wherein said optical mediumis formed of one of silicon, germanium, diamond, III-V semiconductorsincluding GaAs and II-VI semiconductors including ZnSe, silica glass,fluororesin, polyethylene, or polycarbonate-containing organicmaterials.
 8. The reflection-type terahertz spectrometric methodaccording to claim 5, wherein said optical path and said sample arespatially isolated from each other at said interface of said opticalmedium.
 9. The reflection-type terahertz spectrometric method accordingto claim 5, wherein said optical medium is provided, at said interface,with a thin film which has a refractive index smaller than that of saidoptical medium and does not absorb said terahertz waves such that saidterahertz waves undergo total internal reflection at said interface onwhich said thin film is formed.
 10. A reflection-type terahertzspectrometer, comprising: an input optical path through which polarizedterahertz waves are propagated, an irradiating means which irradiates asample with said polarized terahertz waves propagated through said inputoptical path, an output optical path through which polarized terahertzwaves having been exiting from said irradiating means are propagated,and a detecting means which receives and detects said polarizedterahertz waves propagated through said output optical path, wherein:said irradiating means has at least one planar interface and arefractive index greater than that of a peripheral region contactingsaid planar interface and is disposed between said input optical pathand said output optical path such that said polarized terahertz wavespropagated through said input optical path to be incident on said planarinterface undergo total internal reflection at said planar interface,said sample is disposed in said peripheral region contacting said planarinterface of said irradiating means, and when said polarized terahertzwaves undergo said total internal reflection at said planar interface,said sample is irradiated with evanescent waves scattering from saidplanar interface to said peripheral region contacting said planarinterface, so as to measure a spectrum, and the reflection-typeterahertz spectrometer further comprising a polarization control meanswhich controls polarization of said polarized terahertz waves at saidinput optical path, so as to control polarization of said evanescentwaves.
 11. The reflection-type terahertz spectrometer according to claim10, wherein said irradiating means is formed of one of silicon,germanium, diamond, III-V semiconductors including GaAs and II-VIsemiconductors including ZnSe, silica glass, fluororesin, polyethylene,or polycarbonate-containing organic materials.
 12. The reflection-typeterahertz spectrometer according to claim 10, further comprising ahousing with an opening for accommodating said input optical path andsaid output optical path, said irradiating means being disposed so as toclose said opening with said planar interface.
 13. The reflection-typeterahertz spectrometer according to claim 10, wherein a thin film whichhas a refractive index smaller than that of said irradiating means anddoes not absorb said polarized terahertz waves is formed on said planarinterface of said irradiating means.
 14. A reflection-type terahertzspectrometer, comprising: a terahertz wave generating means forgenerating terahertz waves, an input optical path through whichterahertz waves propagate, a polarization control means polarizing theterahertz waves, an irradiating means which irradiates a sample with thepolarized terahertz waves propagated through said input optical path, anoutput optical path through which terahertz waves having been exitingfrom said irradiating means are propagated, and a detecting means whichreceives and detects said terahertz waves propagated through said outputoptical path, wherein said irradiating means has at least one planarinterface and a refractive index greater than that of a peripheralregion contacting said planar interface and is disposed between saidinput optical path and said output optical path such that said terahertzwaves propagated through said input optical path to be incident on saidplanar interface undergo total internal reflection at said planarinterface, and said sample is disposed in said peripheral regioncontacting said planar interface of said irradiating means, and whensaid terahertz waves undergo said total internal reflection at saidplanar interface, said sample is irradiated with evanescent wavesscattering from said planar interface to said peripheral regioncontacting said planar interface, so as to measure a spectrum.
 15. Thereflection-type terahertz spectrometer according to claim 14, whereinsaid irradiating means is formed of one of silicon, germanium, diamond,III-V semiconductors including GaAs and II-VI semiconductors includingZnSe, silica glass, fluororesin, polyethylene, orpolycarbonate-containing organic materials.
 16. The reflection-typeterahertz spectrometer according to claim 14, further comprising ahousing with an opening for accommodating said input optical path andsaid output optical path, said irradiating means being disposed so as toclose said opening with said planar interface.
 17. The reflection-typeterahertz spectrometer according to claim 14, wherein a thin film whichhas a refractive index smaller than that of said irradiating means anddoes not absorb said terahertz waves is formed on said planar interfaceof said irradiating means.
 18. A reflection-type terahertz spectrometer,comprising: a device measuring a spectrum of a sample in a terahertzwavelength region by irradiating terahertz waves from a terahertz wavegenerating means on said sample and detecting reflected waves from saidsample by a detecting means, an optical path placed between saidgenerating means and said detecting means, an optical medium having arefractive index larger than that of said optical path such thatterahertz waves incident on said optical medium undergo total internalreflection at an interface of said optical medium so as to generateevanescent waves from said interface, and a polarization control devicepolarizing said terahertz waves from said generating means to beincident on said optical medium, wherein said sample being placed in avicinity of said interface of said optical medium so as to irradiatesaid evanescent waves on said sample and measure a spectrum.